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  <title>Document</title>
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<body>
  <script>
    // 依次执行异步代码

    // 回调函数依次执行
    function fn1 () {
      return new Promise(resolve => {
        setTimeout(() => {
          console.log(111)
          resolve(1);
        }, 1000);
      })
    }

    function fn2 () {
      return new Promise(resolve => {
        setTimeout(() => {
          console.log(222)
          resolve(2);
        }, 1000);
      })
    }

    function fn3 () {
      return new Promise(resolve => {
        setTimeout(() => {
          console.log(333)
          resolve(3)
        }, 1000);
      })
    }

    // 链式调用  .then方法
    fn1().then(res => {
      console.log(res);
      return fn2();
    }).then(res => {
      console.log(res);
      return fn3();
    }).then(res => {
      console.log(res);
    })


    // 解决回调地狱
    // fn1(function () {
    //   fn2(function () {
    //     fn3(function () {
    //       console.log("所有都执行完毕");
    //     })
    //   })
    // })



    // })
  </script>
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